Herons and Egrets are charismatic symbols of wetlands, and they play important roles in how wetlands and other ecosystems function. Tracking the number of herons and egrets in an area can give us a window into the health of the local environment.
Every year since 1989, a dedicated crew of volunteers has fanned out over the northern San Francisco Bay Area to monitor nesting herons and egrets. They count how many nests are in each colony, and estimate the number of young produced in each nest. Long term monitoring projects like this help use understand how ecosystems change over time as humans continue to alter the landscape and climate continues to warm.
In 2021, we monitored a total of 127 colonies. Of these colonies, Great Egrets nested in 24, Great Blue Herons nested in 44, Snowy Egrets nested in seven, and Black-crowned Night-Herons nested in six.
We can estimate year-to-year changes in the number of birds breeding across the entire region by first calculating the percent change in size of each colony, then averaging those values across all the colonies monitored. These data show that in recent years the local population of Great Egrets is increasing, the local Great Blue Heron population is generally stable, and populations of Snowy Egrets and Black-crowned Night-Herons are generally stable but somewhat more variable year-to year than the other species. You can explore the figures and maps below for more information on each species.
In 2021, we counted 583 Great Egret nests in 24 active colonies. On average, the number of nests in Great Egret colonies increased by 37.4% from 2020. Overall, Great Egret colonies have increased in size during most of this monitoring project. This suggests that the local population of Great Egrets is increasing.
Total Great Egret nesting abundance, 1990 to 2021.
Average annual % change in Great Egret colony size, 1990 to 2021.
Taking a closer look at each individual colony, in 2021, eight Great Egret colonies increased in size, five colonies decreased, and one colony remained the same size.Great Egrets nested in one colony where they did not nest in 2020 (hollow blue circles).There were one active Great Egret colony in 2020 that had zero nests in 2021 (hollow red circles).
Great Egret colonies in 2021.
In 2021, we counted 293 Great Blue Heron nests in 44 active colonies. On average, the number of nests in Great Blue Heron colonies increased by 6.7% from 2020. Overall, the number of nests in Great Blue Heron colonies has increased on average over the last several years, although often not by very much. This suggests that the local Great Blue Heron population is generally stable.
Total Great Blue Heron nesting abundance, 1990 to 2021.
Average annual % change in Great Blue Heron colony size, 1990 to 2021.
Taking a closer look at each individual colony, in 2021, 12 Great Blue Heron colonies increased in size, 10 colonies decreased, and four colonies remained the same size.Great Blue Herons nested in two colonies where they did not nest in 2020 (hollow blue circles).There were two active Great Blue Heron colonies in 2020 that had zero nests in 2021 (hollow red circles).
Great Blue Heron colonies in 2021.
In 2021, we counted 244 Snowy Egret nests in seven active colonies. On average, the number of nests in Snowy Egret colonies decreased from 2020 by 13.2%.The number of nests in Snowy Egret colonies has generally been stable during our monitoring project, suggesting an overall stable population. However, there have been occasional single year spikes in colony growth, which we don’t yet fully understand.
Total Snowy Egret nesting abundance, 1990 to 2021.
Average annual % change in Snowy Egret colony size, 1990 to 2021.
Taking a closer look at each individual colony, in 2021, one Snowy Egret colony increased in size, two colonies decreased, and zero colonies remained the same size.Snowy Egrets nested in one colony where they did not nest in 2020 (hollow blue circles).
Snowy Egret colonies in 2021.
In 2021, we counted 280 Black-crowned Night-Heron nests in six active colonies. On average, the number of nests in Black-crowned Night-Heron colonies decreased from 2020 by 20.5%. In recent years, the number of nests in Black-crowned Night-Heron colonies has tended to increase and decrease more than the other species we monitor. This means that although the population appears generally stable over the long term, there can be dramatic changes in the number of nesting birds from one year to the next.
Total Black-crowned Night-Heron nesting abundance, 1990 to 2021.
Average annual % change in Black-crowned Night-Heron colony size, 1990 to 2021.
Taking a closer look at each individual colony, in 2021, one Black-crowned Night-Heron colony increased in size, two colonies decreased, and zero colonies remained the same size.Black-crowned Night-Herons nested in one colony where they did not nest in 2020 (hollow blue circles).
Black-crowned Night-Heron colonies in 2021.
On this map, the size of the circle represents colony size (number of nests). You can zoom in and pan, and hover over each colony to get more information.